In transportation facilities for tourists, such as an airplane, a receiver device is provided at each of passenger seats in the facilities for providing various services, such as videos (e.g. movies and satellite broadcasting), audios (e.g. music and news), and interactive contents (e.g. games and shopping). These services are provided by connecting a server storing the videos, audios, and interactive contents to deliver them to each of the passenger seats via a transmission network with communications using various network technologies (e.g. streaming technologies, and file transfer protocols).
A movie service particularly in these services provides the audios supplied from each seat, and the visuals supplied from a large screen on a wall. To provide this service, both of a receiver device at each seat for outputting the audios and a projector for projecting the visuals on the wall necessarily have internal time synchronized with each other and output the visuals and audios at the same timing.
In order to provide this service, the internal time of the receiver devices, which is generally asynchronous with each other, are necessarily synchronized with each other. However, in a wired network, routing or other processes in the transmission line may cause time-series random time delay variations, hence preventing a synchronous system using this route from having accurate synchronization.
Japanese Patent Laid-Open Publication No. 2004-129009 discloses a conventional technology for synchronizing the internal time of plural receiver devices with each other. This technology utilizes a reference frequency transmitted from a satellite so as to synchronize the internal time of a transmission source with the internal time of the receiver devices. Japanese Patent Laid-Open Publication No. 11-88305 discloses a method of synchronizing the internal time by delivering reference time via another route, such as satellite communication, causing no delay variation. Alternatively, a network time protocol (NTP) can synchronize the internal time on wired network technologies, such as IEEE802.3 series, or wireless network technologies, such as IEEE802.11 series.
The method disclosed in Japanese Patent Laid-Open Publication No. 2004-129009 requires a large system, such as a satellite, and a communication route just for synchronizing the time other than those for transmitting the visuals and audios.
The method disclosed in Japanese Patent Laid-Open Publication No. 11-88305 requires a large system, such as a satellite, and dedicated circuits.
Systems on the NTP may cause variations in delay time particularly with using bridge devices, hence not ensuring synchronization. Further, the plural receiver devices request NTP timestamps from few NTP servers, thus causing congestion.